Role of Ionic Charge Density in Donnan Exclusion of Monovalent Anions by Nanofiltration

The main objective of this study is to examine how the charge densities of four monovalent anions - fluoride (F^-), chloride (Cl^-), bromide (Br^-), and nitrate (NO₃^-) - influence their Donnan (charge) exclusion by a charged nanofiltration (NF) membrane. We systematically studied the rejection behavior of ternary ion solutions containing sodium cation (Na⁺) and two of the monovalent anions as a function of the pH with a polyamide NF membrane. In the solutions containing F^- and Cl^- or F^- and Br^-, F^- rejection was higher than Cl^- or Br^- rejection only when the solution pH was higher than 5.5, suggesting that F^- (which has a higher charge density) was repelled more strongly by the negatively charged membrane. The order of change in the activation energy for the transport of the four anions through the polyamide membrane as a response to the increase of the membrane negative charge was the following: F^- > Cl^- > NO₃^- > Br^-. This order corroborates our main hypothesis that an anion with a smaller ionic radius, and hence a higher charge density, is more affected by the Donnan (charge)-exclusion mechanism in NF. We conclude with a proposed mechanism for the role of ionic charge density in the rejection of monovalent anions in NF.